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SIM PlanetQuest will exploit the classical measuring tool of astrometry (interferometry) with unprecedented precision to make dramatic advances in many areas of astronomy and astrophysics." (1) In order to achieve the required pointing precision of SIM, a gimbaled Siderostat (SID) pointing mechanism with coarse and fine stage actuators is being employed. In order to better understand the coarse pointing accuracy, a team of engineers set out to design, build, and test a linear ballscrew actuator that can perform submicron incremental steps for 10 years of continuous operation. Point tracking and low disturbance requirements lead the team to implement a ballscrew actuator with a direct drive DC motor and a piezo brake. By using an off the shelf motor, Hall effect sensor, ballscrew, and glass scale encoder, repeatable 20 nm incremental steps (actuator resolution) over a 120mm range was achieved. The results exceed expectations by a factor of 50 times and prove linear nanometer positioning requires no gears, levers, or hydraulic converters.12

This paper describes the development of robust, multi-variable H ∞ control systems for the conversion of the High-Speed Autonomous Rotorcraft Vehicle (HARVee), an experimental tilt-wing aircraft. Tilt-wing rotorcraft combine the high-speed cruise capabilities of a conventional airplane with the hovering capabilities of a helicopter by rotating their wings at the fuselage. Changing between cruise and hover flight modes in mid-air is referred to as the conversion process, or simply conversion. A nonlinear aerodynamic model was previously developed that captures the unique dynamics of the tilt-wing aircraft. An H ∞ design methodology was used to develop cruise and hover control systems because it directly addresses multi-variable and robust design issues. The development of these control systems was governed not only by performance specifications at each particular operating point, but also by the unique requirements of a gain-scheduled conversion control system. The cruise and hover control designs form the basis for the conversion control system. The performance of the resulting conversion closed-loop systems is analyzed in the frequency and time domains. A tilt-wing rotorcraft Modeling, Simulation, Animation, and Real-Time Control (MoSART) software environment provides 3D visualization of the vehicle's dynamics.The environment is useful for conceptualizing the natural rotorcraft dynamics and for gaining an intuitive understanding of the closed-loop system performance. I. INTRODUCTIONTilt-Wing Aircraft Basics. A tilt-wing is an aircraft that has the ability to hover similar to a helicopter while retaining the high-speed cruise capabilities of a conventional propeller-driven airplane. These two extremely different modes of flight are accomplished by tilt-wing vehicles through the rotation of the wing. The wing is maintained in a position perpendicular to the fuselage major axis for hover flight and in a position parallel to the fuselage major axis for cruise flight. The process of moving between hover ( Figure 1a) and cruise (Figure 1b) flight modes is referred to as the conversion process, or simply conversion. Operation between the extremes of cruise and hover is referred to as mid-conversion flight.